Marfan syndrome (MFS) is an autosomal-dominant systemic connective tissue disorder that affects 1 in 5000 individuals. Patients with MFS typically develop aortic root aneurysms with ensuing aortic dissection or rupture remaining the leading cause of death. Previous studies have demonstrated that the underlying fibrillin-1 gene mutation in MFS increases the activity of transforming growth factor-? (TGF-?). Although TGF-? blockade inhibits aortic root aneurysm development in murine models of MFS, the molecular mechanism by which TGF- ? signaling leads to aneurysm development remains unknown. MicroRNAs (miRNAs) are short non-coding single-stranded RNAs that function to regulate hundreds of genes. Our laboratory has reported that miR-29b plays a key pathogenic role in early aneurysm development in a Marfan mouse model (Fbn1C1039G/+). miR-29b is known to regulate genes involved in apoptosis and ECM deposition/remodeling. Importantly, how TGF-? increases miR-29b expression in a manner that is temporally and spatially restricted to the aortic root during early postnatal life, is not clear. The aorta is a heterogeneous structure composed of three layers, the intima, media and adventitia. Lineage studies have shown that vascular smooth muscle cells (SMC) in the different anatomic segments of the aorta have distinct embryological origins. Recent studies have suggested that the diversity of SMC origin may explain site specific location of aneurysm development. Our laboratory has developed induced-pluripotent stem (iPS) cells from human MFS patients and have successfully differentiated them into SMC from different embryologic origins (second heart field, neural crest cells and paraxial mesoderm). This in vitro model system will allow us to study how SMC origin influences regional differences in aneurysm formation. Complementary experiments will be performed using an in vivo Marfan animal model, thus increasing the translational potential of these studies. The overall working hypothesis that differences in the embryonic origin of smooth muscle cells lead to distinct miR-29b-mediated extracellular matrix remodeling and contributes to aortic root aneurysm formation in Marfan syndrome will be tested by the following two specific aims: AIM 1: Determine if the miR-29b upstream regulators and downstream targets are different in Marfan human induced pluripotent stem cell (iPSC)-derived vascular SMCs from different embryologic origins. AIM 2: Contrast the role of miR-29b in the development of aneurysms within aortic segments originating from distinct embryonic origins utilizing the Fbn1C1039G/+ (aortic root; second heart field) and Ang II ApoE (abdominal; mesoderm) mouse models. This proposal will provide a mechanistic framework explaining aneurysm development in MFS. Investigating the pathways that participate in extracellular matrix (ECM) degeneration in the aortic root specifically might translate into more anatomically directed therapeutics.